Not long ago, scientists discovered that swans, the beloved symbols of romantic and sexual fidelity, have some chronic philanderers among their number. (How swans had kept this from us for so long is a mystery.) Other species regarded as paragons of sexual constancy—prairie voles and shingleback skinks—have also proved, on closer inspection, to be inconstant lovers. For the makers of anniversary greeting cards, and for anyone else seeking a precedent in nature for the great human experiment in monogamy, only a handful of mascots remain: black vultures, owl monkeys, California mice.

We know that humans are bad at being faithful, but exactly how bad is hard to tell. Estimates of the number of people who fool around on their partners range, unhelpfully, from less than twenty per cent to more than seventy per cent. Reliable data are scarce, partly because cheaters tend to be untrustworthy on the subject of their cheating, and partly because people disagree on what qualifies as a cheat. Few survey respondents are likely to follow President Carter’s example and include sins of the imagination in their personal inventories; most, it can be assumed, will reject President Clinton’s wishful insistence that oral sex doesn’t count. But, when it comes to interactive porn sessions, or sexting, or occasional snogs with attractive co-workers, one person’s grievous betrayal is another’s harmless hobby.

Notwithstanding the problems of definition and the vague statistics, the consensus among social scientists is that the incidence of infidelity has been rising in recent decades. This is mostly attributed to the fact that modern life has increased and democratized the opportunities for illicit sex. Women, whose adulterous options have historically been limited by domesticity and economic dependence, have entered the workforce and discovered new vistas of romantic temptation. (Men are still the more unfaithful sex, but their rates of infidelity appear to have remained steady over the past three decades, while, according to some estimates, female rates have risen by as much as forty per cent.) Senior citizens have had their sexual capacities indefinitely prolonged by Viagra and hip-replacement surgery. Even the timid and the socially maladroit have been given a leg up, courtesy of the online pander. Adultery may still be, as Anthony Burgess described it, the “most creative of sins,” but, thanks to Tinder et al., engineering a tryst requires significantly less ingenuity and craft now than at any other time in human history.

Surprisingly, perhaps, our increasingly licentious behavior has not been reflected in more tolerant public attitudes toward infidelity. While we’ve become considerably more relaxed about premarital sex, gay sex, and interracial sex, our disapproval of extramarital sex has been largely unaffected by our growing propensity to engage in it. We are eating forbidden apples more hungrily than ever, but we slap ourselves with every bite. According to a 2017 Gallup poll, Americans deplore adultery (which is still illegal in some two dozen states and still included among the crimes of “moral turpitude” that can justify denial of citizenship) at much higher rates than they do abortion, animal testing, or euthanasia.

The fact that a prohibition is often violated is not an argument, per se, for giving up on the prohibition. Humans kill one another with some frequency, and we continue to believe that our laws against murder are a good idea. If we keep failing to meet our own standards, the solution, some would suggest, is simply to try harder. The couples therapist and relationship guru Esther Perel believes otherwise. In her new book, “The State of Affairs: Rethinking Infidelity” (Harper), she argues that we would be better off coming to a more compassionate accommodation of our unruly desires. Decades of administering to adulterers and their anguished spouses have convinced her that we need “a more nuanced and less judgmental conversation about infidelity,” one that acknowledges that “the intricacies of love and desire don’t yield to simple categorizations of good and bad, victim and culprit.” Our judgmental attitude toward our transgressions does not make us any less likely to commit them, she argues—“infidelity has a tenacity that marriage can only envy”—and it keeps us from understanding why we transgress. The desire to stray is not evil but human.

Traditional couples therapy focusses on the defense and enforcement of the monogamous pact, and tends to side firmly and explicitly with the faithful spouse. He or she is often referred to as “the injured party,” while the straying partner is labelled “the perpetrator.” The standard assumption is that an affair is a symptom either of marital dysfunction or of some pathology on the part of the perpetrator. (Sex addiction and fear of intimacy are the most common diagnoses, although lately a genetic predisposition to infidelity has been gaining traction.)

This approach, Perel believes, does little justice to the “multifaceted experience of infidelity.” It demonizes adulterers, without pausing to explore their motives. It focusses on the traumatic effects of affairs, without acknowledging their “generative” possibilities. “To look at straying simply in terms of its ravages is not only reductionistic but also unhelpful,” she writes. Affairs can be devastatingly painful for the ones betrayed, but they can also be invigorating for marriages. If couples could be persuaded to take a more sympathetic, less catastrophic view of infidelity, they would, she proposes, have a better chance of weathering its occasional occurrence. When people ask her if she is against or in favor of affairs, her standard response is “yes.”

Perel, who is Belgian-born but practices in New York, is much sought after for her sophisticated, European-flavored insights into love and desire, and she has made a specialty of challenging the puritanical orthodoxies of the American therapy industry. “Mating in Captivity” (2006), the book that brought her to public notice, was a sprightly disquisition on the anaphrodisiac effects of married life, in which she argued that the excessive value placed on communication and transparency in modern relationships tends to foster conjugal coziness at the expense of erotic vitality. Her suggestion that couples seeking to sustain their élan vital would do well to cultivate a little distance and mystery was not original, or particularly radical, but it inspired wariness and even hostility among some of her colleagues, who felt that she approached the solemn project of saving American marriages with insufficient reverence.

The new book, which expands on (and occasionally repeats) the ideas explored in the last, has met with similar objections. Perel has been accused of trivializing the scourge of infidelity and of promoting ideas that are fundamentally hostile to the institution of marriage. It’s difficult, however, to find any real evidence for these charges. Perel is more sanguine than others about the capacity of a marriage to withstand adulterous lapses, but her belief in coupledom—her commitment to the idea of commitment—is never in doubt. Insofar as she stresses the importance of flexibility, patience, and even stoicism in long-term relationships, her book bears a distinctly traditional message.

Perel takes a very stern line on what she sees as the excessive sense of entitlement that contemporary couples bring to their relationships. Their outsized expectations of what marriage can and should provide—perpetual excitement, comfort, sexual bliss, intellectual stimulus, and so on—together with their callow, “consumerist” approach to romantic choices, leave them ill-equipped to cope with the inevitable frustrations and longueurs of the long haul. They are too quick to look elsewhere the moment that their “needs aren’t being met,” and too ready to despair the moment that the promise of sexual loyalty is broken. Those who show willingness to forgive infidelity risk being chastised by friends and relatives for their lack of gumption. Women, Perel notes, are under particular pressure these days to leave cheating spouses as a mark of their feminist “self-respect.”

One reason, of course, that crises of infidelity attract such vampiric interest is that they lift the peacetime ban on judging other couples’ complex relations. For a moment, the wall of privacy around a marriage is breached and everyone gets to peer in and make assessments. The outrage and moral certainty expressed on such occasions can be comforting for the betrayed spouse, but they are largely “unhelpful,” according to Perel. In order to come to any adult reckoning with an affair, the betrayed must avoid wallowing too long in the warm bath of righteousness. For a period immediately following the revelation, a certain amount of wild rage and sanctimony is permissible, but after that the rigorous work of exploring the meaning and motives of an affair must begin.

The scrupulous evenhandedness of Perel’s approach is eminently reasonable in theory. She wants to redress a traditional bias against cheating spouses, to acknowledge “the point of view of both parties—what it did to one and what it meant to the other.” In practice, it must be said, her method seems to demand heroic levels of forbearance on the part of faithful spouses. They are asked not only to forgo the presumption of their own moral superiority but to consider and empathize with what has been meaningful, liberating, or joyous about their partners’ adulterous experiences. The affair that has caused them so much anguish may have been prompted by boredom or a longing for sexual variety, or it may have been a bid for existential “growth, exploration, and transformation.” (It’s hard to imagine anyone being gladdened by the news that his or her spouse’s adultery was an Odyssean quest for self-discovery.)

They are also asked to control their vengeful impulses, learning to “metabolize” their desire for vengeance “in a healthy manner.” (A healthy act of vengeance is making your spouse send a check to your favorite charity, not sewing shrimp into the hems of his or her trousers.) They must resist the desire to “know everything” and avoid demanding details about the physical acts involved in their partners’ betrayals. (They can ask “investigative questions” about feelings but not “detective questions” about hair color, sexual positions, or the size of genital organs.) Americans, Perel observes, are particularly inclined to believe that a process of forensic confession is a necessary forerunner to the restoration of trust, but “coming clean,” she argues, is often more destructive than it is salutary, and “honesty requires careful calibration.”

If you can gird yourself to comply with these guidelines, you have a chance, Perel claims, not only to save your relationship but to transform “the experience of infidelity into an enlarging emotional journey.” Roused from sexual complacency by the threat of a third party, you may find that the sexual spark in your marriage has been reignited. “There is nothing like the eroticized gaze of the third to challenge our domesticated perceptions of each other,” she writes. Now “the ongoing challenge” for you and your partner is to maintain the flame. Tips for doing so include arranging candlelit date nights at home and creating secret e-mail accounts for “private, X-rated conversations during meetings, playdates, and parent-teacher conferences.”

It’s not fair to pass judgment on such ideas. Other people’s efforts to jazz up their flagging marital sex lives are bound to seem a bit grim on the page. Still, in the long list of difficult demands that Perel makes on the human spirit—not seeking revenge, understanding your spouse’s desire to feel “alive” with someone else, and so on—the labor of fending off sexual boredom and keeping domestic life “hot” may strike some as the most punishing and arduous of all.

Perel, who understands the wilting effect of the word “work” in the sexual context, prefers to talk about the need for playfulness and creativity, but the effort involved in the monogamous enterprise cannot be denied. Why is it that when old couples announce how long they have been married people always clap, as if the pair had completed a particularly gruelling race or survived cancer? What is being applauded if not their endurance, their masochistic rigor? Home fires are apt to lose some of their ferocity in the long term, no matter how much creativity is expended on keeping them alight. Might it not be better to stop fetishizing sexual exclusivity as the sine qua non of happy relationships?

Perel is not unsympathetic to this thought, and, toward the end of her book, she devotes a brief chapter to various forms of consensual non-monogamy. She writes about couples who swing, couples who have chosen to be, in the term coined by the sex columnist Dan Savage, “monogamish,” and couples who have expanded into “triads,” “quads,” or “polyamorous pods.” (Those interested in a more comprehensive taxonomy of such arrangements may wish to consult “It’s Called ‘Polyamory,’ ” by Tamara Pincus and Rebecca Hiles, a book that provides definitions of, among other things, “designer relationships,” “relationship anarchy,” and the polyamorous “Z.”) Perel praises the efforts of all these non-monogamists “to tackle the core existential paradoxes that every couple wrestles with—security and adventure, togetherness and autonomy, stability and novelty,” and she is careful to remind the squeamish that many of these “romantic pluralists” succeed in maintaining rather higher standards of loyalty and honesty than do their monogamous counterparts.

She remains, however, appropriately skeptical about whether any relationship construct, no matter how cunningly or thoughtfully devised, can offer permanent solutions to the dilemmas of romantic love. The polyamorist aspiration to replace sexual jealousy with “compersion” (a delight in one’s partner’s sexual delight with someone else) is just that: an aspiration. People often end up in open relationships out of a desire to propitiate restless lovers, rather than through any interest of their own—with predictably miserable results. And no amount of expanding or softening the boundaries of fidelity will ever outwit the human desire to transgress. The conventional bourgeois marriage invites adultery. The earnest polyamorous setup, in which every new lover is openly acknowledged and everyone’s feelings are patiently discussed at Yalta-type summits, invites some more imaginative trespass: not using a condom, or introducing the lover to your parents. “In the realm of the erotic,” Perel writes, “negotiated freedom is not nearly as enticing as stolen pleasures.”

This—the impossibility of absolute romantic security—is the bracing moral at the center of Perel’s book. There is no “affair proof” marriage, she warns, whatever the self-help industry tries to tell you. To love is to be vulnerable. Relationships can inspire varying degrees of trust, but trust is always, as the psychoanalyst Adam Phillips puts it, “a risk masquerading as a promise.” To believe yourself to be the sole progenitor of your partner’s desire, rather than merely its current recipient, is a folly. Elizabeth Hardwick, who stoically endured the countless infidelities of her husband, Robert Lowell, knew something about this. In her famous essay “Seduction and Betrayal,” she described the terrible wisdom vouchsafed to the betrayed heroine of classic literature: she “is never under the illusion that love or sex confers rights upon human beings. She may of course begin with the hope, and romance would scarcely be possible otherwise; however, the truth hits her sharply, like vision or revelation when the time comes. Affections are not things and persons never can become possessions, matters of ownership. The desolate soul knows this immediately and only the trivial pretend that it can be otherwise.” ♦

This article appears in the print edition of the December 18 & 25, 2017, issue, with the headline “Infidels.”

Zoë Heller contributes to The New York Review of Books. She has published three novels, including “Notes on a Scandal.”